The first step in understanding a transmission system is to define the basic broadband transmission unit. For analog systems, this would be a channel group, which prior to transmission is typically multiplexed into super-groups and mastergroups. For digital transmission, the basic unit is the digroup (digital group) carried in the DS1 signal. The DS1 signal developed by a digital channel bank (e.g., the D-3 Channel Bank, as disclosed in U.S. Pat. No. 4,059,731, issued Nov. 2, 1977 to J. H. Green and J. E. Landry) and transmitted over a T-1 transmission line (1.544 megabits per second) is, at present, the workhorse of the Bell System digital transmission network.
The format of the DS1 digital signal consists of 24 eight-bit words and one framing bit for a total of 193 bits per frame. The 24 words typically represent 24 separate and distinct message deposited in 24 separate and distinct channels. The words are PCM (pulse code modulation) encoded and the least significant bit (i.e., eighth bit) of a channel is periodically dedicated (every sixth frame) for signaling purposes.
Now over the past several years there has been increasing interest in achieving a more efficient digital encoding. For an evolving digital network, a most interesting application is the possible replacement of the 64,000 bit-per-second (bps) PCM signal (8 bits per channel, repeated at an 8 kHz rate) for telephony. The reason, of course, is to achieve bandwidth compression, and thus a concomitant increase in transmission capacity. To this end, the patent application of D. W. Petr, Ser. No. 343,355 filed Jan. 27, 1982, now U.S. Pat. No. 4,437,087 issued Mar. 13, 1984, discloses an efficient, robust, bit compression algorithm. In accordance with the Petr invention, each 64 Kbps (DSO) signal is converted or compressed to a 32 Kbps signal thereby doubling the capacity of a T1 line, for example.
While the Petr coding algorithm reduces the bit rate for the DSO (64 kbps) channels by half, the bit rate required for signaling is potentially doubled since the number of channels to be carried on a specific facility is potentially doubled. In the DS1 digital signal the last significant bit of a channel is periodically preempted for signaling purposes with little degradation in performance. However, periodic preemption of one of the four bits of a 32 kbps coded channel could significantly impair performance. Furthermore, many of the applications which could use 32 kbps coded channels require that the signal be compatible with network switching elements such as the Digital Access and Cross-connect System (DACS), which have the capability of switching or cross-connecting DSO channels (or integral multiples thereof) while maintaining the association of the message channel and signaling therein.
DACS was designed to operate on DSO channels and the signaling scheme described previously. The introduction of 32 kbps channels in networks containing DACS poses a compatibility problem, the solution of which falls to the equipment which implements the 32 kbps coding.
The CCITT A-Law System adopted by most European administrations has a digital signal format that consists of 30 service channels, one channel devoted to framing bits together with alarm bits, and one channel devoted to signaling. Here again, however, to increase the transmission capacity of this digital signal would result in a signaling problem due to the increased bandwidth required for signaling associated with the added channels.